X-ray tubes for medical diagnostic equipment typically make use of the inventions as claimed and described in U.S. Pat. No. 2,121,631, U.S. Pat. No. 2,336,271, U.S. Pat. No. 2,863,083 and U.S. Pat. No. 2,942,126 or similar applications. Conventional X-ray tubes for high power operation typically comprise an evacuated chamber which holds a cathode filament through which a heating or filament current is passed. A high voltage potential, usually in the order between 40 kV and 160 kV, is applied between the cathode and an anode which is also located within the evacuated chamber. This voltage potential causes electrons emitted by the cathode to be accelerated in the direction of the anode. The emitted electron beam then impinges on a small area (focal spot) on the anode surface with sufficient kinetic energy to generate X-ray beams, the latter consisting of high-energetic photons ejected by said anode, which can then e.g. be used for medical imaging or material analysis. The interaction of the electron beam and anode requires to use high-Z focal track materials, such as tungsten and tungsten-rhenium alloys.
However, it should be noted that this method of X-ray generation is extremely inefficient, which is due to the fact that most of the electric power which is applied to an X-ray tube is converted into heat and because one of the most important power limiting factors of nowadays high power X-ray tubes is the melting temperature of the employed anode material. Conversion efficiency from electron beam power to X-ray power is at maximum between about 1% and 2%, but in many cases even lower. Consequently, the anode target of a high power X-ray tube carries an extreme heat load, especially in the range of the anode target's focal spot, a relatively small target area sub-surface volume covering a surface area with a size of about a few square millimeters, which would lead to the destruction of the anode if no special measures of heat management were taken.
Efficient heat dissipation thus represents one of the greatest challenges faced in the development of current high power X-ray tubes. At the same time, a small focal spot size is required for high spatial resolution of the imaging system, which leads to very high energy densities at the focal spot. Therefore, tube designs are usually highly tailored for heat dissipation and thermal management capability, notably by high speed rotation of the anode about a fixed cathode and by the use of temperature control (via high thermal conductivity and emissivity) bulk materials and coatings. In particular, conventional thermal management techniques for X-ray anodes as known from the prior art may include                using materials that are able to resist very high temperatures,        using materials that are able to store a large amount of heat, as it is difficult to transport the heat out of the vacuum tube,        enlarging the thermally effective focal spot area without enlarging the optical focus by using a small angle of the anode, and        enlarging the thermally effective focal spot area by rotating the anode.        
Except for high power X-ray tubes with a large cooling capacity, using X-ray tubes with a moving target (e.g. a rotary anode) is very effective. It relies on thermal conduction and radiation as thermal transport mechanisms since convection does occur in the evacuated tube. Compared to stationary anodes, X-ray tubes of the rotary-anode type offer the advantage of quickly distributing the thermal energy that is generated in the focal spot such that damaging of the anode material (e.g. melting or cracking) is avoided. Rotation thereby allows for thermal conduction and radiation to avoid local melting of the anode target area. This permits an increase in power for short scan times which, due to wider detector coverage, went down in modern CT systems from about 30 seconds to 3 seconds. The higher the velocity of the focal track with respect to the electron beam, the shorter the time during which the electron beam deposits its power into the same small volume of material and thus the lower the resulting peak temperature.
High focal track velocity is accomplished by designing the anode as a rotating disk with a large radius (e.g. about 20 cm) and rotating this disk at a high frequency (e.g. at more than 150 Hz). However, as the anode is rotating in a vacuum, the transfer of thermal energy to the outside of the tube envelope depends largely on radiation, which is not as effective as the liquid cooling used in stationary anodes. Rotary anodes are thus designed for high heat storage capacity and for good radiation exchange between anode and tube envelope. The problem of dissipating the heat from a rotary anode tube is of such major importance that it has received attention over a period of many years and various methods for obtaining rapid dissipation of heat have been suggested and presented in the relevant literature.
Another difficulty associated with rotary anodes is the operation of a bearing system under vacuum and the protection of this system against the destructive forces of the anode's high temperatures. In the early days of rotary anode X-ray tubes, limited heat storage capacity of the anode was the main hindrance to high tube performance. This has changed with the introduction of new technologies. For example, graphite blocks brazed to the anode may be foreseen which dramatically increase heat storage capacity and heat dissipation, liquid anode bearing systems (sliding bearings) may provide heat conductivity to a surrounding cooling oil, and providing rotating envelope tubes allows direct liquid cooling for the backside of the rotary anode.
The first use of a rotary anode X-ray tube provides the basis for further improvements in the apparatus, one of which is provided in the present invention. The earliest use of a rotary anode in an X-ray tube is provided in U.S. Pat. No. 1,893,759, issued in January 1933. What is described is a rotary anode, therein referred to as an anti-cathode, which comprises a tungsten conical rod, hollowed to allow attachment to a copper sleeve and two ball bearings and rotating about a copper inner rod. All the essential features of the rotary anode X-ray tube are already provided in this prior-art document: a) encapsulation of the X-ray emitting device into a single glass enclosure, b) use of a tungsten cathode, c) use of a rotary anode (anti-cathode) to allow for higher X-ray emission by virtue of avoiding local heating that otherwise occurs on a stationary anode, d) a two-bearing axial attachment of the anode to copper (Cu) for external heat transfer, and e) incorporation of a copper cylinder to form the motor stator for rotation. In this early invention, the motor has entirely encapsulated in vacuo by the glass enclosure.
The concept of an inlayed focal track in the rotary anode member is e.g. described in U.S. Pat. No. 1,977,275. The apparatus involved tungsten (W) or molybdenum (Mo) incorporated in a copper alloy sleeve to increase heat transfer over a single piece of tungsten. The apparatus employs a copper-graphite alloy to provide in vacuo lubrication to the two bearing system. Sliding bearings of the graphite-containing copper alloy are used rather than the previous invention with ball bearings to reduce the noise level of the device containing ball bearings. The rotary anode target is formed with the inlayed focal track by heat shrink fitting the copper alloy sleeve onto the bearing assembly, the latter containing a bolted joint cylinder with the copper alloy sleeve bearings. Current devices have returned to ball bearings to realize a much greater surface velocity associated with high-speed rotation of the anode target. Present devices incorporate other lubricating means, such as silver (Ag) and lead (Pb) coatings onto the bearing elements prior to X-ray tube assembly, most often the balls. A bolted joint connection between the anode target and bearing assembly is also a common feature in current practices (see e.g. U.S. Pat. No. 5,498,187).
As described above, initial inventions for rotary anode target in X-ray tubes, such as e.g. U.S. Pat. No. 2,121,631, utilized an all-refractory metal target for maximizing the X-ray generation while exploiting the high melting temperature of this class of metals. However, it is undesirable to use only one refractory metal (e.g. tungsten) or its alloys as the anode target as a result of high cost, extreme room temperature brittleness, and high density.
This is particularly the case for a tungsten anode which maximizes the relative X-ray photon generation by virtue of a high atomic number Z.
Several inventions lead to improvements in the anode target to reduce overall weight, cost, and dramatically increase the photon flux from the X-ray generation source by increased target radius (hence focal track circumference), heat dissipation capability, and effective increases in the lifetime of the apparatus. Concomitant improvements in other sections of the X-ray tube design (e.g. cathode, use of novel materials) have allowed achievement of these goals.
X-ray anode targets used in present day Computerized Tomography (CT) medical imaging scanners utilize the same basic invention of the rotary anode configuration with a fixed tungsten filament cathode, but rely on an anode target disk of a titanium-zirconium-molybdenum (TZM) alloy containing a continuous track of a tungsten-rhenium (W/Re) alloy towards the outer anode radius. TZM alloys satisfy several critical design requirements for the anode X-ray target without relying on a single tungsten-rhenium alloy structure: a) relatively high strength, b) high melting temperature, c) rapid thermal conduction of heat from the electron beam impingement upon the W/Re track with high kinetic energy provided by a potential difference of about 100 kV, d) electrical conductivity, and e) large mechanical loads caused by rotation at 10,000 rpm and gyroscopic acceleration and de-acceleration loads on the CT scanner gantry.
Improvements in cardiac imaging require the use of higher speed CT gantry rotation, below 0.3 seconds per revolution. This translates into faster speed of the rotary anode target to exceed 30,000 rpm, which is not attainable with the prior art since overloading occurs for a variety of components in the X-ray tube; namely, anode target, target attachment, and cantilever bearing system. Reducing the weight of the anode target reduces the load for each of these issues and may permit even faster gantry scanning rates, subsequently higher target rotation speeds. A carbon-carbon composite is favored for a light-weight anode target material since it has very low density, high specific strength, high temperature use capability and successful use in demanding load and elevated temperatures applications. Nominal physical and mechanical properties of carbon-carbon composites are listed in Table 2 at room temperature (r.t.) and elevated temperatures (see ASM International, ASM Engineered Materials Reference Book, 2nd Ed., 1994).
The application of carbon-carbon composite structures allows to combine the knowledge and experience from previous rotary anode X-ray target designs with the use of carbon-carbon composites in fields other than diagnostic medical imaging. Previous developments are separated here for convenience into (a) development and invention of the substrate material, and (b) adherent protective coatings for carbon composites. Specifically, the development of carbon-carbon composite substrate materials in which carbon-fiber reinforced carbon matrix composites were first developed for rocket components (cf. Buckley, J. D., Edie, D. D., Carbon-Carbon Materials and Composites, Noyes Publications, 1993) and later commercialized as high-friction/low-density materials for aircraft brakes (see Windhorst, T. and Blount, G., Materials and Design, 18[1] (1997) 11). Coating of carbon composites is a major materials development goal for carbon composite coatings to provide high temperature oxidation resistance for the reinforcement fibers and carbon matrix and for component attachment. Metal alloys and inorganic compounds have been utilized for this purpose, providing prior art applicable to the development of carbon composites for anode targets. Coating of carbon composites is taught for use in a wide variety of applications requiring reliable operation in extreme conditions, such as e.g. rocket nozzle components, fusion reactor containment walls and other critical components, microwave tubes, heat exchangers, and submarine hull designs.
An adherent refractory metal coating to carbon composites forms the focal track area for X-ray generation for the rotary anode and is of vital importance in the application of carbon-based substrates for use in X-ray tubes. We also learn key aspects of the previous anode design described above in the prior art and apply it to the use of carbon-carbon composites for a rotating X-ray anode substrate, namely: a) bonding of a thin focal track material onto solid metal targets, b) bonding of a refractory metal onto a solid graphite target, and c) bonding of a graphite ring onto a molybdenum alloy cap. The prior art for coating attachment will be examined here from all available uses and compared with issued patents and pending applications relating to carbon composite materials for rotary anode X-ray targets.
In U.S. Pat. No. 6,554,179, the focal track attachment issue is directly addressed for the X-ray tube application with a carbon-carbon composite substrate. Green-state slurries of powder layers are applied to the carbon composite and fired at high temperature to achieve a tailored interface with a refractory metal top layer as the focal track. The bonding layers include carbides or borides of hafnium (Hf) and zirconium (Zr) powders, combined with these powders or thin foils in elemental forms. The process in the preferred embodiments involves formation of a layered stack followed by a single high temperature firing step in a vacuum or inert gas: a) application of the initial powder slurry containing hafnium or zirconium carbides or borides with hafnium or zirconium powder, b) drying at 125° C., c) addition of a hafnium or zirconium thin foil or powder, d) added power layer of refractory metal such as e.g. tungsten (W) and molybdenum (Mo) for the focal track, e) light compaction pressure, and f) firing for at least fifteen minutes at high temperatures for densification. U.S. Pat. No. 6,554,179 teaches that including hafnium and zirconium powder incorporated in the carbide or boride slurry lowers the sintering temperature to a temperature between 1,700° C. and 1,900° C. from higher temperature firing at 2,350° C. with slurry devoid of the elemental powders. In contrast, one form of the embodiment as described in U.S. Pat. No. 6,554,179 involves high temperature firing at 2,350° C. of interlayers followed by a second 2,350° C. firing with the additional of focal track powders applied at the top surface.
U.S. Pat. No. 5,943,389 addresses the need for a carbon-carbon composite substrate through a hybrid approach of using a graphite substrate and attaching a high thermal conductivity array of carbon fibers embedded in a multilayer stack for mitigating the thermal expansion mismatch between the focal track and carbon materials. This involves using a forest of about 10% to 40% volume of thin chopped carbon fibers perpendicular to a carbon substrate, and embedded in several functional layers: a) bonding layer between the fiber ends and the carbon substrate (although it remains undetermined as to the best method for the alignment and attachment procedure), b) rhenium overcoating of the carbon fibers to form a 3 μm to 5 μm diffusion barrier to the high-Z focal track materials, and c) a mixture of tungsten (W), tungsten-rhenium (W/Re), hafnium carbide (HfC), tantalum carbide (TaC), zirconium carbide (ZrC) and niobium carbide (NbC) to fill between the coated carbon fiber and overlay a continuous layer which incorporates the carbon fiber array. The high-Z elements, alloys and carbides are varied to accommodate the thermal expansion mismatch between the carbon substrate, fiber composite layer, and high-Z focal track. High-thermal conductivity carbon fibers with a diameter between 8 μm and 12 μm and having a length between 0.003 inches and 0.030 inches (which means between about 80 μm and 800 μm) are used in the preferred embodiment.
Although U.S. Pat. No. 5,943,389 teaches to incorporate short fiber composites into a layer with tailored thermal expansion materials, there is not disclosed any method of fiber placement and attachment to the carbon substrate; a particularly important issue since carbon fibers are commonly available in tows consisting of at least 10,000 fibers. Rhenium (Re) is chosen in U.S. Pat. No. 5,943,389, as the carbon-diffusion barrier attached to the carbon fibers is a stated reason of expected low solubility of carbon in rhenium, thermal matching with the carbon fiber and small decrease in thermal conduction from the focal track to the fiber array. Fundamentally, rhenium is more likely to be a good choice for the interlayer since there is rhenium carbide formation at the focal track temperatures exceeding 2,000° C. The conversion rate to rhenium carbide remains unknown but can be determined in time-temperature exposure experiments, and the X-ray photoelectron spectroscopy (XPS) depth profile of a thin rhenium foil bonded at high temperature to a carbon substrate in vacuum and under a low load.
In U.S. Pat. No. 6,430,264, the use of a carbon-carbon composite as a light-weight rotary anode target is described as well as the design and method for producing the focal track. Distinction is made from the carbon-carbon composite with existing designs with a TZM cap and graphite storage ring and with use of graphite as the anode target substrate. A carbon-carbon composite allows for a light-weight target to achieve higher accelerations and X-ray flux than feasible with a TZM/graphite target. Although use of a graphite substrate is also light-weight, it is pointed out that the strength of graphite is not sufficient for use as a substrate material at the speeds and accelerations needed in future CT systems. A carbon fiber reinforced carbon matrix substrate is preferred and cited in the claims as a result of light weight, high strength, thermal conductivity and current availability produced by chemical vapor deposition and infiltration methods. Attachment of the focal track to the carbon-carbon composite is described as following a roughing procedure for the annular region of the substrate in which the focal track materials are to be attached. One embodiment describes the use of a 1-2 μm layer of tantalum (Ta) followed by a 30 μm thick layer of rhenium (Re), and overcoating of the tantalum and rhenium layers with the tungsten-rhenium (W/Re) alloy of 0.010 inch (250 μm) thickness. Tantalum is selected as the interface to the carbon-carbon composite substrate, since it is a carbide forming compound at the focal track temperatures and owing to the required duration of use. It is envisaged that the entire tantalum layer will be converted to tantalum carbide (TaC) and provide a useful bonding layer between the focal track alloy and carbon-carbon anode substrate. Bonding will be further promoted by using a relatively thick layer of rhenium between the tantalum (hence converted to tantalum carbide) interlayer and tungsten-rhenium (W/Re) track. This provides for a carbon-diffusion barrier.
Although the science is not part of the claims in U.S. Pat. No. 6,430,264, we learn from prior art that the tungsten carbide forms a weak interface to a carbon-carbon composite substrate, and is to be avoided for a practical anode target, both in article fabrication and through the lifetime of the device where a measurable reaction rate between materials is likely. A rhenium interlayer is described in several previous inventions of a carbon-based anode target, such as e.g. in U.S. Pat. No. 3,579,022. Furthermore, U.S. Pat. No. 6,430,264 also cites the use of a single tantalum layer with a relatively large thickness (˜10 μm) to form the focal track after conversion at high temperature to tantalum carbide. Several other carbide-forming bonding layers are provided in U.S. Pat. No. 6,430,264 (cf. claim 11) to have the same affect as a thin layer of tantalum (Ta)—the preferred embodiment—between the carbon substrate and a tungsten-rhenium focal track: hafnium (Hf), zirconium (Zr), niobium (Nb), titanium (Ti) and vanadium (V) along with their alloys.